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<p>Reactive iron oxides (Fe<sub>R</sub>), often termed the “rusty sink” of organic carbon (OC), has been well studied in estuarine settings but remains poorly constrained in deep-sea sediments. Here, we investigate reactive iron-bound OC (Fe<sub>R</sub>-OC) dynamics in cold seep sediments from the Qiongdongnan Basin, northern South China Sea, with emphasis on the role of anaerobic oxidation of methane (AOM). The relative contribution of Fe<sub>R</sub>-OC to total OC (<i>f</i><sub>FeR-OC</sub>) ranged from 5.2 to 40.0% (mean: 15.7% ± 8.1%) and decreased from reference sites to methane-rich sediments, primarily due to iron oxide sulfidization and subsequent pyrite formation. Methane-derived OC accounted for 0–11.1% of TOC (2.2 ± 2.6%) and 0–12.7% of Fe<sub>R</sub>-OC (3.8 ± 4.1%), with higher values in sulfate-methane transition zones (SMTZs) than in non-SMTZ layers. These results indicate that sulfate-dependent AOM is a major source of methane-derived OC, which associates with Fe<sub>R</sub> through adsorption and co-precipitation, partially offsetting the decline in <i>f</i><sub>FeR-OC</sub>. The greater contribution of methane-derived OC to Fe<sub>R</sub>-OC than to bulk TOC, together with more depleted δ<sup>13</sup>C<sub>FeR-OC</sub> relative to δ<sup>13</sup>C<sub>TOC</sub>, supports the preferential binding of methane-derived OC by Fe<sub>R</sub>. Importantly, Fe<sub>R</sub> and Fe<sub>R</sub>-OC in methane-rich sediments were not primarily controlled by bulk Fe content but rather by TOC source and the Fe<sub>R</sub>-OC/Fe<sub>R</sub> values. Collectively, our findings reveal dual processes of Fe<sub>R</sub>-OC preservation—loss through sulfidization and retention via methane-derived OC—and underscore the significance of AOM-driven iron–carbon coupling for carbon sequestration in deep-sea environments.</p>

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Characterizing iron-bound organic carbon in cold seep sediment cores: impacts of anaerobic oxidation of methane

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年4月17日

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  • Yang Wu,
  • Qianyong Liang,
  • Huachun Liu,
  • Jinpeng Wang,
  • Xi Xiao,
  • Yifei Dong,
  • Binbin Guo,
  • Xin Ni,
  • Jing Zhao

摘要

Reactive iron oxides (FeR), often termed the “rusty sink” of organic carbon (OC), has been well studied in estuarine settings but remains poorly constrained in deep-sea sediments. Here, we investigate reactive iron-bound OC (FeR-OC) dynamics in cold seep sediments from the Qiongdongnan Basin, northern South China Sea, with emphasis on the role of anaerobic oxidation of methane (AOM). The relative contribution of FeR-OC to total OC (fFeR-OC) ranged from 5.2 to 40.0% (mean: 15.7% ± 8.1%) and decreased from reference sites to methane-rich sediments, primarily due to iron oxide sulfidization and subsequent pyrite formation. Methane-derived OC accounted for 0–11.1% of TOC (2.2 ± 2.6%) and 0–12.7% of FeR-OC (3.8 ± 4.1%), with higher values in sulfate-methane transition zones (SMTZs) than in non-SMTZ layers. These results indicate that sulfate-dependent AOM is a major source of methane-derived OC, which associates with FeR through adsorption and co-precipitation, partially offsetting the decline in fFeR-OC. The greater contribution of methane-derived OC to FeR-OC than to bulk TOC, together with more depleted δ13CFeR-OC relative to δ13CTOC, supports the preferential binding of methane-derived OC by FeR. Importantly, FeR and FeR-OC in methane-rich sediments were not primarily controlled by bulk Fe content but rather by TOC source and the FeR-OC/FeR values. Collectively, our findings reveal dual processes of FeR-OC preservation—loss through sulfidization and retention via methane-derived OC—and underscore the significance of AOM-driven iron–carbon coupling for carbon sequestration in deep-sea environments.